Bone screw retaining system

ABSTRACT

An implant, particularly for the spinal column, comprises a plate having openings, bone-anchoring members capable of being accommodated in the openings and at least one split-ring capable of holding the members in the orifices. The split-ring can come into direct contact with the anchoring member or members to hold the member or members in the orifices.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a divisional of U.S. Ser. No.09/665,530 filed Sep. 19, 2000.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to osteosynthesis devices for thespinal column, the devices comprising a plate and a mechanism forlocking a bone screw or anchoring member in position. This divisionalapplication relates to tools used to remove a bone screw after it hasbeen locked in the plate.

[0003] U.S. Pat. No. 5,876,402 relates to an osteosynthesis platecomprising through-holes of conical shape capable of housing a bonescrew with a completely spherical head to which is clipped, so as toform a ball-joint connection. A split coupling element of conicalexterior shape complementing that of the hole is provided. A circlipreduces the aperture of the through-hole. A similar clip and groovearrangement is shown in U.S. Pat. Nos. 5,879,389 and 6,102,952.

[0004] In U.S. Pat. No. 5,876,402, the bone screw is placed in thecoupling element prior to insertion in the plate. Upon insertion, thesplit coupling element opens up the circlip. This circlip closes upagain once the coupling element has passed through. The coupling elementis thus held captive in the through-hole. Final clamping of theanchoring member in position is achieved by the frictional wedging ofthe coupling element in the bottom of the cone.

[0005] In such a system, the number of parts makes the clamping-inposition of the anchoring members weak. In addition, the clamping doesnot occur at the instant when the circlip closes up again after thepassage of the coupling element. This leads to the risk of the assemblybecoming unclamped, which is prejudicial to the patient.

[0006]FIGS. 5 and 6 of U.S. Pat. Nos. 5,879,389 and 6,102,952 show asplit-ring for installation in a groove after the bone screw or anchorhas been installed in the bone.

SUMMARY OF THE INVENTION

[0007] One of the objects of the present invention is to provide aspinal implant which is easier to fit while at the same time beingreliable.

[0008] With a view to achieving this objective, the present inventionenvisages an implant, particularly for the spinal column, comprising ajoining member such as a plate exhibiting openings or orifices,bone-anchoring members such as bone screws capable of being accommodatedin the orifices and at least one split ring capable of holding themembers in the orifices. The split ring can come into direct contactwith the anchoring member or members to hold the member or members inthe orifices.

[0009] Thus, the number of parts involved in the locking is reduced andthis locking can be made more reliable. Advantageously, the joiningmember comprises a plate and the orifices comprise an opening with aspherical seat.

[0010] Preferably, each anchoring member or bone screw comprises acomplementary spherical part capable of coming into contact with thespherical seat. Thus, the surgeon has, at his disposal, freedom toorient the anchoring member angularly with respect to the joining memberor plate, thus allowing him to optimize the anchorage.

[0011] Advantageously, the anchoring members or bone screws comprisedriving means such as a drive socket.

[0012] In one embodiment the split ring is preferably common to at leasttwo orifices and includes a driving means, which driving means compriseopenings. In another embodiment, the split ring is specific to eachorifice in the plate.

[0013] Advantageously, the split ring has a variable cross-section so asto optimize its flexibility. Thus, the ring will deform more readilywhen introducing the head into the orifice. The amount of time taken andthe number of operations required during surgical intervention will bereduced.

[0014] The bone plate, screw and ring may be supplied as part of a screwlocking system for bone plates to be used by a surgeon. The bone platehas at least one opening therein, and normally a plurality of openings,for receiving a bone screw or bone anchor. The openings extend along anaxis from a top surface to a bottom bone contacting surface of theplate. Each opening has an upper region with a first diameter with agroove formed therein having a depth defined by a diameter greater thanthe first diameter. The plate has a lower region including a seat forthe bone screw. The bone screw has a head with a maximum diameter whichis smaller than the first diameter, thereby allowing the screw head topass through that region of the opening.

[0015] An expandable ring is provided which is pre-mounted in the grooveand having, when relaxed and unexpanded, an external diameter greaterthan the first diameter, but smaller than the groove diameter. Theexpandable ring has an internal diameter when relaxed and unexpanded,smaller than both the first and the head diameters. The expandable ringis capable of expanding into the groove so that the internal diameterexpands to be larger than or equal to the screw head diameter while, atthe same time, the external diameter is less than or equal to the groovediameter.

[0016] With this geometry, the split-ring can be pre-mounted in thegroove and the screw can be inserted, shank first into the bone platefrom the upper non-bone contacting surface and, upon engagement betweenthe head of the screw and the split-ring, the split-ring expands intothe groove, allowing the head to pass therethrough. Once the screw headhas passed through this split-ring, it contracts under its naturalspring tension. When the ring relaxes to its unexpanded state, itprevents the bone screw from backing out of the plate by the engagementof an undersurface of the split-ring and an upwardly facing surface onthe bone screw.

[0017] The openings in the lower portion of the bone plate have a partspherical seat portion located between the groove and the bottom bonecontacting surface of the plate with an opening in the bottom platesurface to allow the shank of the bone screw to pass through. The screwhead has a corresponding part spherical surface extending from the shankof the screw towards the upwardly facing surface of the screw. Uponinsertion of the screw through the plate, the screw head engages thepart spherical seat on the bone plate. At that point the screw head isbelow the split-ring groove. The bone screw shank can be threaded in anywell known fashion and may include an axial groove to enable the screwto be self-boring and self-tapping. The bone screw may include aninternal bore extending along the longitudinal axis of the screw whichincludes threads for engaging a pull out tool should removal of thescrew be necessary.

[0018] In order to enhance the locking system's ability to prevent thescrew from backing out of the bone plate, both the groove and split-ringhave complementary inclined surfaces extending towards the upper surfaceof the bone plate upon moving towards the center of the opening in theradial direction. The engagement of the surfaces in combination with aforce exerted by the screw on the bottom surface of the split-ringcauses the internal diameter of the ring to decrease with increasingforce from below. This insures the bone screw cannot back out of theopening.

[0019] In order to make the insertion of the bone screw easier, it isprovided with an inclined surface complementary to an inclined surfaceon the internal bore of the split-ring, which inclined surfaces increasein diameter upon moving in a direction from the bottom surface of theplate towards the upper surface of the plate and radially outwardly ofthe opening central axis. Thus, when the screw head inclined surfaceengages the complementary inclined surface on the internal diameter ofthe split-ring, forces are generated which expand the split-ring intothe groove. In order to increase the flexibility of the split-ring, atleast one cutout and preferably three or more cutouts are spaced aroundthe external diameter of the ring, resulting in a variablecross-section. This allows the ring to have more flexibility inexpanding than if the external diameter of the ring were constant. Inorder to better prevent the egress of the bone screw from the plate, thesurface of the split-ring facing towards the bottom of the plate is flatand extends generally perpendicularly to the central axis through eachopening. The bone screw has a complimentary upwardly facing generallyflat or slightly inclined surface.

[0020] The location of the groove in the plate is such that when thehead of the screw fully engages the spherical seat in the plate, theupwardly facing surface is located below the bottom surface of thesplit-ring. In order to allow the bone screw to rotate from side to sideonce seated, an angular cutout of 0° to 20° can be provided at thebottom surface of the plate, thereby making the opening on the bottomsurface oblong in at least one direction. This allows the longitudinalaxis of the screw head and shank to be rotated between 0° and 20° withrespect to the central axis of the opening.

[0021] The material for the split-ring must be flexible and becompatible with the body and it has been found that the titanium alloydisclosed in U.S. Pat. Nos. 4,857,269 and 4,952,236, which have modulusof elasticity not exceeding 100 GPa, is acceptable. Polymeric materialssuch as ultra-high molecular weight polyethylene are also acceptable.

[0022] The joining member or plate may be curved to match the anatomicalcurvatures. Thus, the implant curved to best suit the anatomy andnatural curvature of the spinal column in the case of a spinalapplication. Of course, the plate may be used in fracture fixation, as atibial baseplate, as a hip side plate or any application where boneplates and screws are used. For these uses, a larger screw than thatdescribed herein is necessary. The screw locking system can be scaled upfrom that described herein so that any size screw can be utilized in asmaller locking system.

[0023] Also envisaged is a method for implanting the implant involvingaccessing the spinal column via an anterior route, fitting the implant,preparing the anchorage, fitting the anchorage members, locking theimplant and the head of the anchoring members with respect to thejoining member, and closing up the access route.

[0024] These and other objects and advantages of the present inventionwill become apparent from the following description of the accompanyingdrawings. It is to be understood that the drawings are to be used forthe purposes of illustration only and not as a definition of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Other features and advantages of the invention will become moreapparent on reading the description which follows of the preferredembodiments which are given by way of non-limiting examples.

[0026]FIG. 1 is a perspective view of a first embodiment of theinvention;

[0027]FIG. 2 is an exploded perspective view of the first embodiment;

[0028]FIG. 3 is a cross-sectional view along lines III-III of the firstembodiment of FIG. 1;

[0029]FIG. 4a is a plan view from above of the first embodiment;

[0030]FIG. 4b is an elevation view of the first embodiment shown in FIG.4a;

[0031]FIG. 4c is a front view of the first embodiment shown in FIG. 4a;

[0032]FIG. 5 is a perspective view of a second embodiment of theinvention;

[0033]FIG. 6 is an exploded perspective view of the second embodiment;

[0034]FIG. 7 is a partial view in section on the plane VII-VII of thesecond embodiment;

[0035]FIG. 7a is a cross-sectional view of a bone screw or anchor of thepresent invention;

[0036]FIG. 7b is a cross-sectional view of a single orifice used in thesecond embodiment of the invention without the screw and split-ringalong line VII-VII of FIG. 6;

[0037]FIG. 7c is a plan view of the split-ring of the second embodimentof the present invention;

[0038]FIG. 7d is a cross-sectional view of the split-ring of FIG. 7calong lines A-A;

[0039]FIG. 8a is a plan view of the second embodiment of the invention;

[0040]FIG. 8b is an elevation view of the second embodiment shown inFIG. 8a;

[0041]FIG. 8c is a front view of the second embodiment shown in FIG. 8a;

[0042]FIG. 9 is a perspective view of a third embodiment of theinvention;

[0043]FIG. 10 is a plan view of a screw driver for driving the bonescrews of FIG. 7a from the orifice of FIG. 7b;

[0044]FIG. 10a is an end view of the screw driver shown in FIG. 10;

[0045]FIG. 11 is a plan view of an extractor tube for extracting theanchor or bone screw from the plate after implantation;

[0046]FIG. 11a is an end view of the extractor shown in FIG. 11;

[0047]FIG. 11b is an enlarged detail of the drive and of the extractorshown in FIG. 11; and

[0048]FIG. 12 is a plan view of a threaded extraction shaft designed toengage the screw and pull it axially out of the hole, should it beimpossible to unscrew the threaded shank.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] With reference to FIGS. 1 to 4 c, there is shown the implantaccording to the first embodiment comprises a plate 1, bone screws 5 andcirclips or split-rings 4. The plate 1 is a bone plate such as ananterior cervical plate or any other plate designed to be held on boneby bone screws. Plate 1 may join two bone parts or stabilize a fractureor may sit on a resected bone surface such as on a tibial plateau.

[0050] In the preferred embodiment, plate 1 is formed of a body 11ending in two ends 12 which have a width slightly greater than that of amid-zone of body 11. Each of the ends 12 comprises a pair of openings ororifices 2 which pass through the entire thickness of plate 1. The fouropenings are arranged geometrically as at four corners of a rectangle.Each of the openings 2 has a first, upper, cylindrical part 23 whichcontinues in the form of a spherical central part 21 and ends in asecond, lower, cylindrical part 22, the diameter of which is smallerthan that of the first cylindrical part 23. The spherical intermediatepart 21 allows the angle of the bone screw 5 that is to be accommodatedin the opening 2 to be chosen.

[0051] Plate 1 preferably comprises two blind holes 3 which have acircular opening and a recess 31. The two blind holes 3 are arranged onthe longitudinal mid-segment of the rectangle, near the respective pairsof corners. Recess 31 is such that it protrudes into the pair ofopenings 2 to which it is adjacent, thus creating an open slot 32 ineach opening 2 of the pair. This slot 32 is made in such a way that itis located in the first cylindrical part 23 of the openings 2.

[0052] Plate 1 has a first curvature 13 in its longitudinal plane, asdepicted in FIG. 4c. This curvature 13 allows plate 1 to follow thenatural lordosis of the section of spine for which the plate 1 depictedin FIGS. 1 to 4 c, is intended. In addition, the plate 1 has a secondcurvature 14 in its transverse plane as depicted in FIG. 4b. Thiscurvature 14 allows plate 1 to match as closely as possible the shape ofthe body of the vertebra to which it is connected.

[0053] Each recess 31 is capable of housing a circlip 4. The circlip 4is in the form of a circular ring 43 split at 42. The circlip orsplit-ring 4 comprises driving means 41 which, in this embodiment, arelugs projecting towards the inside of the ring. Each lug may be shapedto receive the tips of a pair of needle nose pliers (See FIG. 9).

[0054] Once in place in the recess 31, with the circlip 4 in theposition of rest, i.e. in the open position, it protrudes into the pairof openings 2 adjacent to it through the slot 32 of each opening 2. Itthus closes up the opening 2 slightly.

[0055] The bone screw 5 is the preferred anchoring member in theembodiment which allows the plate 1 to be connected to the bodies of thevertebrae which are fitted with the present invention. The preferredscrew 5 has a head 57 surmounting a cylindrical part or shank with athread 51 suited for bone, comprising a self-tapping means 55 at itsdistal end. These tapping means allow the screw to better penetrate thebone when being driven. The head 57 comprises a drive 52 which, in thisinstance, is embodied by a hexagonal socket. In addition, the head 57comprises a slightly conical part 53 which is continued in the form of apart 56 forming a rim extending towards the outside of the screw 5 andinclined slightly with respect to a plane perpendicular to the axis A ofthe screw 5.

[0056] Finally, the head 57 of the screw 5 ends in a spherical male part54 which complements the female intermediate part 21 of the opening 2and which meets the threaded cylindrical part or shank 51. Thesecomplementing forms allow the bone screw 5 to be set at a chosen anglewith respect to plate 1. The anchoring of plate 1 can thus be optimizedby the surgeon during the operation.

[0057] Preferably, the implant of the present invention shown in FIGS.1-4 is supplied to the surgeon with the two circlips 4 installed inrecesses 31 of plate 1. If the plate is an anterior cervical plate, itis preferably implanted by an anterior access route and by uncoveringthe vertebral bodies that will be fitted. The surgeon positions theplate 1 then pierces pilot holes through each pair of openings where hewishes to have an anchorage. He then engages a bone screw in each pilothole. He screws these in until the part 54 of their head 5 comes intocontact with the part of the ring 44 of the circlip 4 that projectsthrough the orifice 32. At this point, there are two possible options:

[0058] 1. The surgeon closes up the circlip 4 by bringing the two lugs41 closer together using pliers and then, holding the circlip closed, hescrews the two bone screws 5 in until the complementary spherical parts21 and 54 come into contact, then, releasing the circlip which returnsto the open position over the rim 56;

[0059] 2. The surgeon continues to screw in the anchoring member 5, thespherical part 54 pushing the ring 44 into the slot 32 through a rampeffect and thus forming its passage, and the ring will open againautomatically once the rim 56 has passed by, and the complementaryspherical parts 21 and 54 will be in contact.

[0060] Locking is provided by contact between the complementaryspherical parts 21 and 54 and by the re-opening of the circlip 4 abovethe rim 56. The second role of the rim 56 is to limit the possibilitiesof angular orientation. This prevents the screw from coming out of thevertebral body or from coming into contact with its counterpart fittedin the other opening 2 forming the pair. In both instances, the platewould be poorly anchored or even not anchored to the vertebral body atall. Thus, having introduced each screw into the orifice via its distalend, the circlip prevents the screw from backing out of the orifice.

[0061] In the event of an adjustment, the surgeon can easily withdrawthe plate 1 simply by unscrewing the bone screws 5 after having closedup the circlips 4 by moving their lugs 41 closer together, thusuncovering the aperture of the orifice 2.

[0062] In a second embodiment illustrated by FIGS. 5 to 8 c, cervicalplate 1 is preferably still formed of a body 111 ending at two ends 112which are slightly wider than the body 111. Each of the ends 112 stillhas a pair of openings 102 which pass right through the entire thicknessof the plate 101. Each opening 102 has a first part 123 which iscylindrical, then a spherical intermediate part 121. Preferably, theorifice or opening 102 has a part 122 in the form of an angular cutoutin the lengthwise direction of the plate 101. Preferably, the cutoutallows the screw to pivot an angle B, preferably from 0° to 20°, in thelengthwise direction about axis 164, preferably the width of the cutout122 is slightly less than its length. A circular recess or groove 131 isformed in the cylindrical part 123 of each opening 102. As in theprevious embodiment, when used as an anterior cervical plate, the plate101 has a curvature 13 in its longitudinal plane and a curvature 14 inits transverse plane. The roles of these curvatures are the same as inthe previous embodiment.

[0063] The recess 131 is able to accommodate a circlip or split-ring104. As before, the circlip 104 is in the form of a circular ring 143,split at 142. The preferred circlip or split-ring 104 in this instancehas tabs 141 and cutouts 149 distributed uniformly around the entirecircumference of the ring 143. Preferably, there are at least 3 of thesetabs. They make it possible to be sure that the circlip will not escapefrom the groove or recess 131, while leaving thinner parts of the ring143 to allow better flexibility when deforming or expanding the circlipas will be discussed hereinbelow. Of course it is possible to make thering thinner or use other means to achieve flexibility in the ring. Forexample, one or two tabs could be used if the cutouts in the ring aresized sufficiently to produce the required flexibility. The circlip 104comprises expansion chamfer or ramp 144 in the form of an inlet chamferlocated on the interior side 145 of the ring 143.

[0064] The bone screw 105 differs from that of the previous embodimentin that the drive 152 which is in the form of a cross is extended in oneembodiment by a blind bore 158 coaxial with the axis A of the screw 105.This allows the use of a screwdriver with a flat or cruciform bladeextended by a small cylindrical protrusion that complements the blindbore 158. Thus, when tightening or loosening, the screwdriver cannotslip to injure nearby living tissues or irreversibly deform the circlip104, as this would compromise locking.

[0065] The preferred head 157 has a part 153 which is generally conicaland which slightly bows outwardly which is continued radially outward bya part 156 forming an upwardly facing rim surface extending towards theoutside of the screw 105 and which preferably is slightly inclined withrespect to a plane perpendicular to the axis A of the screw 105.

[0066] Finally, a part spherical portion 154 that complements theintermediate part 121 of the opening 102 allows the outer edge of theportion 154 to meet the threaded cylindrical part or shank 151, which isthreaded with a bone-screw thread. The purpose of this complementingnature is to allow the angle of the screw 105 to be chosen with respectto the plate 101 in order to optimize anchorage.

[0067] As in the previous embodiment, the implant is supplied to thesurgeon with the four split-rings or circlips 104 installed in the fourrecesses 131 in the plate 101. As before, the surgeon, having made hisaccess route, then positions the plate 101 and pierces the pilot holesthrough the pairs of openings 102 where he wishes to anchor, completelyscrews in the bone screws 105. At the end of tightening, the sphericalpart 154 will come into contact with the chamfer 144 of the cirelip 104and then, through a bearing action, open up the latter to make itspassage towards the spherical intermediate part 121 of the orifice 102.The circlip 104 will close back up again automatically once the rim 156has passed, and the complementary spherical parts 121 and 154 will be incontact.

[0068] Performing these two operations makes sure that the screw 105 islocked in the plate 101. As before, the second role of the upwardlyfacing rim 156 is to limit the possibilities of angular adjustment. Thisprevents the screw from coming out of the vertebral body or its threadedshank 151 from coming into contact with its counterpart fitted in theother orifice 102 forming the pair. In both instances the plate would bepoorly anchored or not anchored to the vertebral body at all. In theevent of an adjustment, the surgeon can easily withdraw the plate 101simply by unscrewing the bone screws 105 after having opened up circlip104 as will be discussed below.

[0069] A preferred bone screw of the second embodiment of the presentinvention is shown in greater detail in FIG. 7a. In the preferredembodiment, the blind bore 158 of screw 105 is threaded for a portion160 located below drive 152 towards the tip 161 of the screw. Thefunction of the threaded portion will be described in greater detailbelow.

[0070] It should be noted that the preferred screw 105 has a nominalthread diameter of about 4 mm with the outer diameter of the upwardlyfacing surface rim 156 being about 5 mm. If desired, the leading end ortip 161 of the screw shank 151 may include a groove or other structurefor allowing the bone screw to be self-drilling and self-tapping. Inthis situation, no pilot hole need be drilled by the surgeon.

[0071] Referring to FIG. 7b, there is shown the preferred opening 102 inthe bone plate. The recess or groove 131 which accommodates spring clip104 has an upwardly and inwardly inclined surface 133 which, in thepreferred embodiment, extends at an angle of about 20° with respect tothe bottom surface 135 of groove 131. In the preferred embodiment, thebottom surface 135 of groove 131 extends along a plane perpendicular tothe axis 164 of the opening 102. The upper inclined surface 133 isspaced from surface 135 by surface 137 which, in the preferredembodiment, is about 0.3 mm. The maximum diameter to surface 137 ofgroove 131 is, in the preferred embodiment, about 6.9 mm. Spherical seat121 for screw head 157 extends from adjacent the bottom bone contactingsurface of the plate to surface 135. In the preferred embodiment, thespherical surface has a radius of 2.67 mm. Consequently, the partspherical portion 154 of the screw has a similar radius. As can be seenin FIG. 7b, the opening 102 may have an angular cutout along a portionof surface 122 adjacent the bottom plate surface to allow the shank 151of the screw to extend in at least one direction at an angle B ofapproximately 0° to 20° and preferably 10° with respect to the axis 164.Thus, when viewed from the bottom, the opening would appear to be oblongin at least one direction. Of course, the angular cutout can be enlargedto permit angulation in a plurality of directions.

[0072] Referring to FIG. 7c, there is shown a preferred split-ring orcirclip 104 which includes five tabs 141 distributed uniformly aroundthe circumference of the ring 143. In the preferred embodiment, the ringhas an internal diameter 145 of approximately 4.5 mm and a maximumexternal diameter 147 of preferably 6.2 mm. The difference between theexternal diameter 147 and the groove diameter 137 is preferably about0.7 mm. This allows the internal diameter to expand to accommodate thescrew head. The preferred cutouts have a depth of approximately 0.4 mmso that the external diameter 149 at each cutout is approximately 5.4mm. The preferred split 142 is 0.26 mm in width when the split-ring isin its relaxed, i.e. unexpanded condition. The above dimensions aregiven for illustration only and larger screws, openings and split-ringsmay be used in other applications.

[0073] Referring to FIG. 7d, there is shown a cross-section of thesplit-ring shown in FIG. 7c along lines A-A. The split-ring has a bottomsurface 190 oriented to engage the bottom surface 135 of groove 131. Thecross-section has an inclined upper surface 192 for engaging surface 133which is upwardly inclined on moving towards the center the split-ring.Preferably, the incline is at an angle of about 20° with respect tobottom surface 190. The surface forming internal diameter 145 is in twosections, the first is surface 194 which is generally parallel to axis164 of opening 102 and the second is surface 144 which is angledradially outwardly towards surface 192 also at preferably 20° withrespect to surface 194 (and the axis 164). Surfaces 192 and 144 arepreferably connected by a radius 198 rather than a sharp corner. Thepreferred split-ring has an overall height from the surface 190 to thetop of radius 198 of approximately 0.52 mm and the distance alongsurface 196 between surface 190 and 192 is about 0.29 mm.

[0074] The preferred cross-section allows spring-clip 104 to beassembled within groove 131 by the plate manufacturer and shipped to theuser in a pre-assembled condition. It is especially important that theclip 104 have a sufficient number of cutout areas to render itsufficiently flexible for insertion into the inserting recess or groove131 prior to shipping to the end user. It is also necessary to use arelatively flexible material for the ring, which material has a modulusless than 100 GPa. Such a titanium material is found in U.S. Pat. Nos.4,857,269 and 4,952,236. If these titanium alloys are utilized for thesplit-ring, it has been found that advantageous to make the joiningmember or plate and anchoring or bone screw out of the same material,although such is not absolutely necessary. In addition, polymericmaterials can be used for the split-ring. In the preferred embodiment,the split ring 104 has no means for enabling its removal from the grooveafter assembly. Thus, it is not possible for the surgeon to remove thering from the plate.

[0075] Another advantageous feature of the split-ring is the preferably20° incline of the top surface 192 which engages with complementarygroove surface 133. This is advantageous because forces generated fromthe backing out of the screw 105 against the bottom surface 190 ofsplit-ring 104 tend to keep the inner diameter 145 from expanding. Inaddition, only a small annular inter-engagement between the bottomsurface 190 of ring 104 and the upwardly facing surface 156 is necessaryto prevent screw 105 from backing out of hole 2 in plate 1. In thepreferred embodiment, this annular overlap is at least 0.07 mm andpreferably between 0.07 mm on a radius and 0.11 mm.

[0076] In a third embodiment illustrated in FIG. 9, the device 201differs from a first embodiment only in the shape of the blind holes 203and of the circlip 204 that can be housed in the recess 231. The shapeof the holes 203 has a semicircular base 237 continued by two straightsurfaces 236 which converge towards each other and are connected attheir other end of the side of the associated orifices 2 by asemicircular vertex 235 of smaller radius than the base 237. This shapefacilities the fitting of the circlip 204. The latter is very similar tothe one in the first embodiment, except for the lugs 241 which haveholes 243 to take the jaws of a driving instrument. Installation withthis embodiment is identical to that of the first embodiment.

[0077] The circlip 104 may have a constant cross-section.

[0078] The bone screws may be monoaxial: they cannot be oriented withrespect to the plate.

[0079] It can thus be seen that, in the embodiments of FIGS. 1 and 9,one and the same circlip locks two anchoring screws.

[0080] In all these embodiments, each circlip collaborates by directcontact with the screw to prevent it from coming out of the opening,without it being necessary to provide a part acting as an intermediatebetween the circlip and the screw.

[0081] Referring to FIG. 10, there is shown a plan view of the preferredscrewdriver 300 for driving screw 105. The screwdriver 300 includes ahandle 302, a shaft 304 and a drive head 306. Referring to FIG. 10a,there is shown an end view of drive head 306 showing a cruciform drivehaving a pair of mutually perpendicular blades 308. Blades 308 engagedrive 152 on screw 105. In the preferred embodiment, the depth of thecruciform slot forming drive 152 is about 2 mm and the depth of thedrive blades 308 is somewhat less and the width of the four slotsforming drive 152 are about 1 mm with the width of the blades 308 beingslightly less. This geometry ensures excellent engagement between theblades on the driver 300 and the drive 152.

[0082] Referring to FIGS. 11 through 11b, there is shown a tool providedto remove the screw 105 after it has been fully inserted into bone andblocked from backing out by split-ring 104. Referring to FIG. 11 thereis shown an extraction tool 400 having a handle 402 and a tubular driveshaft 404, including a drive tip 406. Handle 402 is also tubular havinga cavity 408 open to an end 410 of handle 402. In the preferredembodiment, the cavity 408 is circular with a diameter of about 8 mm.Inner end 412 of cavity 408 is open to a cannulation 414 which extendsthe length of shaft 404 and through tip 406. In the preferredembodiment, this cannulation is circular with a diameter of about 2 mm.The function of cannulation 414 is described below.

[0083] Referring to FIG. 11b, there is shown an enlarged view of drivetip 406 of extraction tube 400 which, like driver head 306 previouslydescribed, includes a cruciform blade having cross members 416 similarto blades 308. However, the outer diameter of tip 406 is equal to theouter diameter of surface 156 on screw 105. Tip 406 includes an inwardlychamfered portion 418 which allows tip 406 to engage the inner diameter145 of the split-ring and expand it sufficiently to allow the screw tobe unthreaded or pulled back out through inner diameter 145 bycounter-rotation of screw 105 with extractor 400. Once the maximumdiameter of upwardly facing surface 156 passes through the split-ring,it springs inwardly along surface 154 of screw 105.

[0084] It has been found that in some instances, the bone deterioratesso that it is impossible to generate a screw removal force by thecounter-rotation of screw 105 with extractor 400. In this instance,referring to FIG. 12, there is shown an extraction tool 500 designed tofit within the cavity 408 and cannulation 414 of extraction tool 400.Extraction tool 500 includes an upper portion 502, a shaft portion 504,a threaded tip 506 and an enlarged portion 508. The threaded tipincludes threads matching threads 160 in screw 105. In the preferredembodiment, the thread is 1.6 mm in diameter. Thus, when the bone screw105 cannot be removed merely by the counter-rotation of screw 105 withextraction tool 400, extraction shaft 504 is inserted through thecannulation 414 and out tip 406 thereof and into threaded engagementwith threads 160 of bone screw 105. All the surgeon must then do is pullon portion 508 of the extraction tool 500 which pulls screw 105 out ofthe bone.

[0085] Although the invention herein has been described with referenceto particular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A tool for removing a screw from a plate of the type having at leastone bore therein for receiving a screw, the screw having a drive head ata first end and a threaded shank at a second end, the bore extendingalong an axis from an outer surface to a bone contacting surface of saidplate, the bore having a first diameter with a region including arecessed groove with a larger diameter than said first diameter, thescrew drive head having a diameter less than said first diameter, anexpandable ring mounted in the groove and having a relaxed unexpandedinner diameter less than the screw head diameter, the ring capable ofexpanding in the groove on contacting the screw head, so that when theinternal ring diameter expands to at least the screw head diameter, saidscrew head can pass through the ring, said tool comprising: a driveshaft with tip for engaging the drive head of the screw, the tip havinga lead surface for engaging the relaxed internal diameter of said ringand expanding the ring into the groove to open the ring at least to thescrew head diameter so that said screw head may be backed out of thebore.
 2. The tool as set forth in claim 1, wherein said drive shaft iscannulated.
 3. The tool as set forth in claim 2, wherein said screwincludes a threaded bore open to the first screw end alignable with saidcannulation in said drive shaft.
 4. The tool as set forth in claim 3further including a screw puller having a shaft with a threaded tip,said puller shaft slidably received in said cannulated drive shaft withsaid threaded tip extending from an end of said drive shaft for engagingsaid thread in said screw.
 5. The tool as set forth in claim 4, whereinsaid drive shaft has a trailing end including a handle.
 6. The tool asset forth in claim 5, wherein said handle includes an internal cavitycommunicating with said cannulation in said drive shaft.
 7. The tool asset forth in claim 6, wherein said screw puller has an enlarged portioncoupled to said shaft with said threaded tip, said enlarged portionreceived within said cavity in said handle.
 8. The tool as set forth inclaim 1, wherein said drive shaft tip includes a chamfered portion forengaging the inner diameter of the ring.
 9. The tool as set forth inclaim 1, wherein the ring is a split-ring.
 10. The tool as set forth inclaim 9, wherein said drive shaft is cannulated.
 11. The tool as setforth in claim 10, wherein said screw includes a threaded bore open tothe first screw end alignable with said cannulation in said drive shaft.12. The tool as set forth in claim 11 further including a screw pullerhaving a shaft with a threaded tip, said puller shaft slidably receivedin said cannulated drive shaft with said threaded tip extending from anend of said drive shaft for engaging said thread in said screw.
 13. Amethod for extracting a cannulated screw having a drive head at a firstend and a threaded shank at a second end with a threaded bore in thescrew open to the first end, from a bore in a plate, the bore having agroove therein spaced a first distance from an outer surface of theplate, the groove including an expandable ring having an unexpandedinner diameter less than an outer diameter of the drive head, the screwdrive head being located at a second distance from said outer platesurface, said second distance being greater than said first distance,comprising: inserting into said bore, a cannulated drive shaft having aleading end with a first diameter less than the unexpanded diameter ofsaid expandable ring and a second diameter at least equal to thediameter of said screw drive head, said leading end including a driveelement; expanding the ring into said groove with said second diameterof said drive shaft so that said inner diameter of said ring is at leastequal to the outer diameter of the screw drive head; and extracting saidscrew from said bore while said ring is expanded.
 14. The method as setforth in claim 13 further comprising engaging the drive element on saiddrive shaft with a complimentary drive element on said screw androtating said screw.
 15. The method as set forth in claim 13 furthercomprising inserting a rod with a threaded tip though said cannulationin said shaft into threaded engagement with the threaded bore in thescrew; and extracting said screw while said ring is expanded by pullingon said threaded rod with a threaded tip.
 16. The method as set forth inclaim 15, wherein said rod with a threaded tip has a handle and saiddrive shaft has a trailing end in the form of a handle having a cavityfor receiving the handle of said rod.